It is well known to build up an internal representation of a two-dimensional image to be displayed, in terms of discrete graphic segments that potentially overlap in the image. Where these graphic segments are to be displayed as solid bodies, then it is necessary to determine which segment has priority in the event of overlap between two segments. This depth priority issue is generally handled by assigning differing priorities to the various segments and then resolving any overlap conflicts according to the assigned priorities of the segments concerned. The actual resolution process can be approached in a number of ways, the most well known of which embody the so-called "painters algorithm" in some form. In this algorithm, the image (or frequently, each successive image line) is built up from its component segments in reverse priority order so that higher priority segments overwrite lower priority ones; this approach whilst eminently feasible particularly where a display frame buffer is being used, does require considerable duplication of effort in terms of writing areas that are subsequently overwritten.
A general discussion of image construction from multiple segments and of the priority resolution issue, can be found in most standard textbooks on graphics systems such as "Principles of Interactive Computer Graphics" William M. Newman and Robert F Sproull, second edition, McGraw-Hill.
It is also known to provide a degree of association between graphic segments rather than treating each segment in isolation. In particular, it is known to associate segments in a windowing, or containment, relationship whereby one segment (the parent segment) contains one or more further segments (child segments), the latter being clipped to the boundary of the parent segment but overwriting the latter where they overlap. The parent segment thus acts as a window through which its children can be viewed. Generally, though not in all cases, the containment relationship is implemented in such a way that spatial transformations applied to the parent (that is, translation, rotation and scaling) are also experienced by any contained children.
One example of a graphics system in which the graphic segments are inter-related by containment relationships is the system described in the doctoral thesis of the present inventor (Doctoral Thesis, "Computer Display Architecture", Peter Hemingway, Cambridge University Library, England, class mark PHD 150 76). In this system, all graphic segments are organized into a tree structure with the parent-child relationship being a containment relationship. Although the parent-child containment relationship does partly define the relative priorities of the segments, this definition is not complete as the relative priorities of siblings (children of the same parent) must also be defined, this generally being done by the application causing generation of the tree. The containment relationships are implemented in this system such that children experience spatial transformations to which their parents are subject; in addition, each child is subject to a spatial transformation relative to its parent. As a result, the positioning, orientation and size of each segment (or segment portion) appearing in the final image is a concatenation of a chain of spatial transformations extending from the segment up through its ancestors to the root of the segment tree.
Although the graphics system described in the aforesaid thesis offers substantial flexibility, there are certain natural relationships that the system cannot readily represent. Accordingly, it is an object of the present invention to provide a graphics system, and in particular an organization of graphic segments, that permits increased flexibility of association between segments.